1. Field of Invention
The present invention relates to a micro-electro-mechanical system (MEMS) device, especially a MEMS device wherein a top fixed electrode thereof has an extension wall and a bottom fixed electrode thereof has an extension wall, and a total surface area of the top fixed electrode extension wall facing amass and a total surface area of the bottom fixed electrode extension wall facing the mass are substantially equivalent.
2. Description of Related Art
One kind of MEMS device is out-of-plane sensors for sensing an out-of-plane movement. FIG. 1A shows a prior art MEMS out-of-plane sensor 10. The MEMS out-of-plane sensor 10 includes a seesaw 11 having a pivot P. When the MEMS device 10 moves alone a direction A, the seesaw 11 will swing because the pivot P leans to one side of the seesaw 11. Atop electrode E13 disposed above the seesaw 11 and two bottom electrodes E11 and E12 on the substrate 12 form a differential capacitor pair. The displacement of the movement can be calculated according to the differential capacitance of the differential capacitor pair (the capacitance of one capacitor increases and the capacitance of the other capacitor decreases). This prior art has a drawback that it requires a precise seesaw structure which is difficult in manufacture.
FIG. 1B shows another prior art MEMS out-of-plane sensor 20 which includes a mass 21, disposed above a substrate 22 and connected to springs through anchors P. A top electrode E23 on the mass 21 and two bottom electrodes E21 and E22 forms two variable capacitors (moving in the same direction). When the out-of-plane sensor 20 moves alone the direction A, the capacitances of the two variable capacitors will increase or decrease simultaneously. This prior art has a relatively lower sensitivity because it does not use a differential capacitor pair as the previous prior art.
FIG. 1C shows another prior art MEMS out-of-plane sensor 30 which intends to improve the prior art MEMS devices of FIGS. 1A and 1B. The MEMS out-of-plane sensor 30 includes a mass 31 including a top electrode E32 and a bottom electrode E31. The top electrode E32 and a top fixed electrode E34 form a top capacitor Ctop, and the bottom electrode E31 and a bottom fixed electrode E33 form a bottom capacitor Cbot. The MEMS out-of-plane sensor 30 has a differential capacitor structure. When the MEMS out-of-plane sensor 30 moves alone the direction A, the capacitances of the top capacitor Ctop and the bottom capacitor Cbot will change in an opposite way. However, besides the top bottom capacitor Ctop and the bottom capacitor Cbot, the extension wall W between the top fixed electrode E34 and the substrate 32 also form a parasitic capacitor C′ with the mass 31, and this extension wall W also form another parasitic capacitor C″ with the bottom fixed electrode E33. The parasitic capacitors C′ and C″ will cause the capacitances of the top capacitor Ctop and the bottom capacitor Cbot to deviate from the desired values. Further, the capacitance deviations of the top capacitor Ctop and the bottom capacitor Cbot are not equivalent, which can cause an unacceptable sensing error in a worst case.
In view of the aforementioned deficiencies of the prior art MEMS devices, the present invention provides a MEMS device and an out-of-plane sensor which include a differential capacitor structure having a low capacitance deviation, and are easy-to-manufacture.